Add Task.custom_sleep_data

[njsmith]

As discussed in #586

[codecov]

Codecov Report

Merging #616 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #616      +/-   ##
==========================================
+ Coverage   99.31%   99.31%   +<.01%     
==========================================
  Files          91       91              
  Lines       10800    10808       +8     
  Branches      751      751              
==========================================
+ Hits        10726    10734       +8     
  Misses         56       56              
  Partials       18       18

Impacted Files	Coverage Δ
trio/_core/_traps.py	100% <ø> (ø)	⬆️
trio/_core/tests/test_run.py	100% <100%> (ø)	⬆️
trio/_core/_run.py	100% <100%> (ø)	⬆️
trio/_core/_parking_lot.py	100% <100%> (ø)	⬆️

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[njsmith]

I think this is good to go, and I'll rebase #586 on top after it's merged.

[pquentin]

Looks good, but I'm not familiar enough with those low level parts to hit merge right now. Let's wait a bit to see if someone else wants to review this.

[oremanj]

Looks good!